http://en.wikipedia.org/wiki/Cube_mapping
http://www.web3d.org/files/specifications/19775-1/V3.3/Part01/components/env_texture.html
there are 3 types of CubemapTexture nodes:
Composed - 6 url fields for front, back, top, left ...
Generated - inline values for the sides
Image - single url field - here they mention DDS as a possible format and give a link, but don't make it a requirement